The combination of a magnetic resonance tomograph with a positron emission tomograph entails heating problems.
The avalanche photodiodes to be used as PET reception diodes are highly sensitive to temperature fluctuations, and must therefore be kept relatively accurately at an operating temperature. On the other hand, owing to the nature of the system, they must be arranged in immediate proximity to time-variable heat sources, such as the gradient coil and the body coil. Furthermore, there are electronic components that generate a heat loss in their vicinity. Another fact which makes it difficult to cool the avalanche photodiodes effectively is that the PET part of the device must be packaged in a radiofrequency-proof housing, in order to avoid mutual interference of the PET and MR.